- Make Log2 exact for powers of two.
- Fix error tolerance function to make tolerance
  a function of the correct (expected) value.

Fixes #9066.

Change-Id: I0320a93ce4130deed1c7b7685627d51acb7bc56d
Reviewed-on: https://go-review.googlesource.com/12230Reviewed-by: Ian Lance Taylor <iant@golang.org>